Heat sensitive copy and recording sheet



Jan. 19, 1965 M. F. BAUMANN ETAL 3,166,433

HEAT SENSITIVE COPY AND RECORDING SHEET Filed Aug. 1, 1962 2 SUPPORT INVENTORS MICHAEL F. BAUMANN WILLIAM R. LAWTON av [75% mi) 9% H ATTO RN EYS United States Patent 3,16%,433 FEAT SENSETHVE CUPY AND RECORDHNG SHEET Michael F. Bauntann, Hudson, and William R. Lawton, Nashua, N.H., assignors to Nashua orporation, Nashua, Nl-L, a corporation of Delaware Filed Aug. 1, 1962, Ser. No. 213,899 6 Gaines. (til. ll73d.$)

This invention relates to heat marking processes and provides a novel heat sensitive sheet useful for copying and recording uses, and characterized by the ability to form a mark of contrasting color when heated to an activation temperature in the range of 50l50 C. Sheets of this type are commonly employed in thermographic copying processes, in which a sheet is placed next to a graphic original and is exposed with the original to infrared radiation to cause selective heating of the dark areas of the original sufiicient to form a copy thereof on the copy sheet. A detailed description of this process is set forth in US. Patent No. 2,746,896. The sheets of this invention may also be used in recording devices in which marking is accomplished by means of a heated stylus.

The heat marking sheets of this invention make use of the known color forming reaction typical of the reduction of a heteropoly phosphomolybdic complex compound. The action of a strong reducing agen on the phosphomolybdic compound results in the rapid formation of a highly contrasting colored mate-rial and for the present purposes of copying and recording his reducing action is initiated by the localized application of heat resulting in a corresponding localized formation of the contrasting colored materials.

The color forming reduction of the heteropoly phosphomolybdic complex compound re uires a strong reducing agent or extreme reaction cond. ons, such as hi h temperatures, in order to obtain the rapid reaction rates and instantaneous color formation required for practical copy or recording sheets. The latter method of insuring the required reaction and color formation rate, that is, the application of high temperatures, is seldom practical due to the dilatorious eiiect of such conditions on the materials usually employed as base or carrier sheets for the copying media, for example, paper and the like. The necessary alternative is, therefore, the use of strong reducing agents as reactants with the heteropoly phosphomolybdic complex compounds.

Although the strong reducing character of desirable reducing agents is advantageous at the moment of colorforming reaction during the process of copying or recording, it usually produces unfavorable side effects; the one of greatest concern being the lack of storage stability resulting from the premature action of the reducing agent. Earlier invesdgators have found the incorporation of strong reducing agents into a heat marking composition with heteropoly phosphomoly'odic complex compounds to be impractical due to various spontaneous reactions resulting, for example, in undesirable premature color formation, or self-exhaustion of the reducing agent by aerial oxidation. It has hitherto been necessary because of such results that only the less effective weak reducing agents be incorporated into a marking medium with the phosphomolybdic compounds. Greater storage stability of a marking sheet is achieved by using such weak reducing agents; however, copying or recording with such sheets under practical conditions is greatly hampered by the low rate of reactivity and resulting marks lack a desirable degree of clarity and contrast.

We have provided, in the present invention, a means by which potentially highly reactive reducing agents may be intimately combined with reducible, color-forming heteropoly phosphomolybdic complex compounds to form com- 3,l%, l$3 Patented Jan. 19, 1965 positions which are stable and non-reacting under conditions of temperature and humidity usually encountered in areas of production, shipment, and storage; these conditions being referred to in the present description and appended claims as normal conditions. Such normally stable combinations are nevertheless rapidly reactive when heated to temperatures in the nange of about 50-l50 C.

to provide the characteristic heteropoly compound reduction products and are, for this reason, particularly useful in heat-marking sheets for copying, recording, or the like.

It is an object of the present invention to provide a copying and recording sheet which is stable and non reactive at normal storage conditions and which is immediately responsive to applied heat within practical temperature ranges to produce a clearly contrasting mark. It is a further object of this invention to provide a means by which a heteropoly phosphomolybdic complex compound and an active reducing medium therefor may be incorporated in a single stable heat-responsive copy sheet.

We have found that when a heteropoly phosphomolybdic complex compound and an hydroxyphenol compound are combined in an alkaline medium such as provided, for example, by a basic organic amino nitrogen compound, that is, an amine or hydrazine, there instantly results the formation of the characteristics highly colored products of the reduction of the phosphomolybdic compound. The hydroxyphenol compounds which we find useful in our invention include the bisphenols and the hydroxyphenol substituted chromans such as the well known 2' hydroxy fiavans. We have further found that a stable, yet potentially reactive composition may be prepared including such color-forming components if, prior to mixture with the phosphomolybdic compound, the hydroxyphenol compound and the basic amino nitrogen compound are com- While so combined as the co-crystal adduct neither the hydroxyphenol compound nor the basic amino compound is available to effect a reduction of the heteropoly phosphomolybdic compound and these two materials, that is, the co-crystal and the heteropoly compound, may safely exist, without color-forming reaction,in intimate physical contact under normal conditions. Immediately upon heating such a normally stable mixture to'a temperature in the range of'the dissociation temperature of the co crystal adduct, however, there occurs the previously-noted characteristic color-forming reduction of the heteropoly pliosphomolybdic compound. The dissociation of the co crystal adduct apparently releases the basic amino compound and the hydroxyphenol compound in their respective unreacted conditions and as such available to cause the rapid color formation by reduction of the heteropoly compound.

In accordance with our present invention we employ the above-described. classes of materials to prepare copy and recording sheets which are stable and non-discoloring at normal conditions and which immediately respond to selective heating to provide correspondingly selective permanent, highly contrasting colorations or markings. The production of heat-marking sheets according to the present invention will typically consist of intimately dispersing a selected co-crystal adduct and heteropoly compound in one or more coatable compositions and applying the same to the surface of a flexible support, such as paper. When dried, the coating will provide the paper with a normally stable heat-responsive layer useful in copy and recording processes.

Solid co-crystal adducts of the type herein described, that is, heat-dissociable adducts containing bisphenols, or hydroxy fiavans, and basic organic amino nitrogen compounds such as amines and hydrazines, are well known. Many bisphenol adducts are described in U.S.

Patent 2,829,175 and in the 'co-pending application of William R. Lawton and Eugene F. Lopez, Serial No. 808,- 012, filed April 22, 1959, now Patent No. 3,076,707, assigned to the assignee of this invention, while hydroxy fiavan adducts are shown by Cramer, Einschlussverbindungen (Springer-Verlag, Berlin, 1954) and in the copending application of William R. Lawton, Serial No. 206,186, filed June '29, 1962.

The co-crystal adducts which we prefer to use are those which are lightly colored, thereby ensuring lighter backgrounds and maximum contrasts for colored marks, and which, in addition, exhibit a dissociation temperature in the range of about 50150 C. when in a coated composition with a heteropoly phosphornolybdic complex compound, thus providing a marking sheet responsive to temperatures in the practical operating range of about 50150 C. We have found that the flavan adducts, as a group, exhibit dissociation temperatures in a generally lower portion of the stated desired temperature range and will be more beneficially employed where lower temperatures are necessary or where higher marking or copying speeds are desired. We additionally prefer to use basic organic amino nitrogen compounds which are normally liquid, since the natural fluid characteristics of such compounds provide a maximum degree of intimate admixing of reactants after activation and dissociation of the co-crystal adduct, and thus ensure complete reaction and color formation.

While the foregoing description relates generally to the application of our invention, the following example will point out in more detail a representative embodiment. In this and subsequent examples all references to formulation parts will be understood to be parts by weight.

Example I A co-crystal adduct is formed by combining:

Parts Tetrachlorobisphenol A (TCBA) 183 Hydrazine 32 Toluene 364 by dissolving the TCBA in the toluene at about 150 F., adding the hydrazine and stirring for about 30 minutes. The mixture is chilled to about 30 F. and held at that temperature for about 14 hours (overnight) during which time a precipitate forms. This is filtered and the precipitate is washed thoroughly with n-hexane at room temperature (70 F.) and dried. The solid crystal product is the co-crystal adduct of tetrachlorobisphenol A and hydrazine having a melting point of 142-146 C. A dispersion of this co-crystal adduct is prepared by ball milling parts of the adduct with 90 parts of a 2.5% solution of polyvinyl butyral (Butvar 13-76) in trichloroethylene. The copy sheet is prepared by coating a lb. per ream (24" x 36"-500) glassine paper to a dry coating weight of 1.0 lb. per ream (24" x 36"500) with a 10% solution of 9 parts phosphornolybdic acid and 1 part citric acid dissolved in ethyl alcohol (denatured). This coating when dried is coated with a dry weight coating of 2.0 lb. per ream (24" x 36"500) of the TCBA/hydrazine co-crystal adduct dispersion. The resulting sheet is suitable for use in thermographic copying processes or heated-stylus recording equipment. Hydra zine derivatives such as phenylhydrazine are also effective in place of the hydrazine.

Ordinarily any of the numerous co-crystal adducts containing a basic organic amino nitrogen compound and an hydroxy flavan may be substituted directly for a hisphenol adduct of the same amino compound in our heatresponsive compositions with comparable results; that is to say, the copy or mark appearing after proper activation of either coated composition will exhibit an equally desirable degree of density, clarity. As previously noted with regard to the respective dissociation or activation temperature ranges of the bisphenol and flavan adducts, however, there are some properties of the adducts that vary with the type of hydroxyphenol compound. The solubility characteristics of the adducts are particularly notable in this respect where it will be observed that fiavan adducts as a group are relatively water-insensitive while the bisphenol adducts are often dissolved and dissociated by water or aqueous compositions. In contrast to this property it has been observed that the comparative resistance of the hydroxy flavan adducts to organic solvents is less'than that of the bisphenol adducts. By prudent selection of one or the other of the bisphenol or hydroxy flavan adducts of the basic organic amino nitrogen compound a wide variety of manufacturing and operating conditions may be realized. For example, a fiavan adduct, due to its Water-insensitivity, may be preferred in manufacturing operations requiring aqueous vehicles or in the preparation of a copy or recording sheet expected to encounter relatively high humidity during periods of storage and use. Such water or solvent resistance is of course necessary to prevent dissociation of the co-crystal adduct, and resulting color reaction, during manufacture or storage and prior to intended time of activation.

When employing aqueous systems in the preparation of our heat-responsive sheets we prefer to use the hydroxy flavan adducts of the basic organic amino nitrogen compound. The following example presents a representative embodiment of our invention incorporating the hydroxy fiavan adducts. It will be noted that the heatresponsive coatings of our sheets may he applied other than as a single layer and thus the normal advantages of multiple coating layers may additionally be enjoyed in the practice of our invention. l/Ve find that the general incompatibility between organic and aqueous systems may be employed in this manner to provide further stability for the heat-responsive sheet product. In order to make use of these and other advantages we prefer the multiple coating method for the application of the flavan adduct compositions.

Example 11 A hydroxyphenol substituted chroman, namely 2'-hydroxy-Z,4,4,7,4'pentamethylfiavan is prepared according to a procedure described by Baker and Besley in J. Chem. Soc., 1951, 1103-6. A mixture or" m-cresol (440 g.) and acetone is saturated with gaseous HCl without cooling and kept at 40 C. for two days with the exclusion of water. The dark product is added to an excess of 20% sodium hydroxide solution, a layer of ether is poured on the surface, and the mixture is vigorously stirred causing the ether addition product of the dimeride to crystallize. After-standing overnight in an open vessel the solid is collected, washed thoroughly with water, dissolved in hexane, and dried with calcium chloride. The solution is filtered and a small amount of ether is added to thesolution. The other complex of the fiavan compound separates in large, colorless, thick rhombic prisms having a melting point of 7 677 C. A fiavan co-crystal adduct is prepared by mixing fifteen parts of a 10% hexane solution of the flavan ether complex with 7.5 parts of dicyclohexylamine, chilling the mixture overnight, and filtering the solid crystalline adduct product, M.P. 103105 C. A coating composition is prepared by dispersing 20 parts of the co-crystal adduct in 100 parts of a 2.0% solution of polyvinyl alcohol in water. The mixture is ball milled to obtained a desired particle size and coated on a sheet of 25 lb. per ream (24" x 36"500= sheets) glassine paper to a dry weight of about 4 lb. per ream. When dried, this layer is coated with about 4 dry lb. per ream of a ball milled dispersion of a mixture of 10 parts of ammonium phosphornalybdate and parts of a 2.5% solution of polyvinyl butyral. The resulting sheet is suitable for use in thermographic copying processes or heated-stylus recording instruments.

In many instances the reducing action of the basic medium produced by the dissociation of the co-crystal adduct may be enhanced by including in the coating composition materials which normally exhibit very limited reducing properties, but which have substantial reducing potential in the basic medium. We have found materials of this type to include ascorbic acid, tartaric acid, lactic acid, and the like and salts of these acids. Another class of materials which is similarly effective lH-lmPIOVlDg the color-forming reduction of the heteropoly compounds in the basic medium of the dissociated co-crystal adduct includes complexes formed from the combination of strong reducing tin salts, such a stannous chloride and stannous sulfate and alcohol amines, e.g., ethanolamine, diethanolamine, methyl ethanolamine, and the like. Apparently these latter complexes are themselves decomposed by the basic components of the co-crystal adducts upon dissociation of the adducts and thereafter contribute to the colorforming reduction of the heteropoly compound.

Example III A cocrystal adduct is formed by combining:

Parts Tetrachlorobisphenol A (TCBA) 183 Ethylene diamine (EDA) 30 Toluene 364 in the manner described in Example I. The solid crystal product is the co-crystal adduct of tetrachlorobisphenol A and ethylene diamine (TCBA/EDA) having a melting point of 208210 C. Dispersions of the following materials are separately prepared by ball milling parts of each material in 90 parts of a 2.5% solution of polyvinyl butyral (Butvar B-76) in trichloroethylene:

(a) Co-crystal adduct TCB A/ EDA (b) Ammonium phosphomolybdate (c) Ascorbic acid The dispersions are milled at 40 F. for about 14 hours (overnight). Varying with the material, such milling Will generally be continued only as long as necessary to obtain a dispersion of optimum particle size.

The resulting dispersions are combined in a weight ratio of about two parts of ammonium phosphomolybdate to one part each of the remaining dispersed solid materials. The mixture thus formed is coated on a 25 lb. per ream (24" x 36"'500 sheet) glassine base to a dry coating weight of about 2.0 lb. per ream. This sheet is suitable for making copies of graphic originals by commercial thermographic means and for use in preparing records in heat-stylus recording instruments.

Example IV A co-crystal adduct is formed by combining:

. Parts Tetrachlorobisphenol A (TCBA) 183 Diethanolamine (DEA) 105 Toluene 364 in the manner described in Example I. The solid crystal product is the co-crystal adduct of tetrachlorobisphenol A and Diethanolamine (TCBA/DEA) having a melting point of l5l153 C. A tin salt complex is prepared by combining:

. Parts Stannous chloride (anhydrous) "190 Triethanolamine (TEA) 150 Water 1800 (c) Ammonium phosphomolybdate (d) Citric acid The dispersions are combined in a Weight ratio of two parts of ammonium phosphomolybdate to one part each of the remaining dispersed solid materials. A 2.0 lb. per ream (24" x 36--50O sheets) dry weight coating of the mixture on a 25 lb. per ream glassine carrier sheet produces a copy sheet suitable for use in thermographic copying processes or in heated-stylus graphic recording instruments.

In the foregoing examples the bisphenol used was tetrachlorobisphenol A (4,4'-isopropylidene-bis-(2,6-dichlorophenol). While this material is one which we have found to be effective in the formation of numerous of the cocrystal adducts which exhibit dissociation temperatures within the presently desirable range of about 50-150 C., it is by no means the only available or useful bisphenol. Many other bisphenol compounds may be employed in the preparation of solid dissociable co-crystal adducts with basic organic amino nitrogen compounds useful in the present invention. Among the preferred bisphenol compounds are included tetrabromobisphenol A; 4,4'-isopropylidenebisphenol; 2,2-methylene-bis-(4-rnethyl-6-butylphenol); 4,4'thio-bis-(6-tert. butyl-m-cresol); 4,4'-butylidene-bis-(6-tert. butyl-m-cresol); 2,2'-thio-bis-(4,6-dichlorophenol), 2,2 methylene bis (3,4,6 trichlorophenol). Other bisphenol compounds which may be used in this manner are indicated in the aforementioned US. Patent 2,829,175.

Numerous hydroxyphenol substituted chromans may be used to prepare the fiaven adducts of the basic organic amino nitrogen compound and among those which we prefer are 7,4'-diethyl2-hydroxy-2,4,4 trimethyl flavan; cyclohexanespiro 4 (2 hydroxy 7,4 dimethyl 2,3- tetramethylene) fiavan; 2'-hydroxy-2,4,4,6,7,4,5-hepta methyl fiavan; 2'-hydroxy-2,4,4,6,5-pentamethyl flavan; 4-p-hydroxyphenyl-2,2,4-trimethyl chroman.

The basic organic amino nitrogen compounds useful in our present invention have been generally set forth as being represented by the amines and hydrazines. In addition to those representative materials noted in the foregoing examples We list here but a few of the many practical amino compounds such as diisopropanolamine, diethylamine, benzylamine, morpholine, cyclohexylamine, diamylamine, di-n-butylamine, and the like. The selection of any of the amino compounds for use in our heatresponsive compositions will depend primarily or individual preferences and desired activity and may be made by means of ordinary experimentation.

In addition to the heteropoly phosphomolybdic complex compounds described above, other suitable compounds may be used, e.g., sodium phosphomolybdate, potassium phosphomolybdate, and the like.

' It is anticipated that specific additives may be incorporated in our compositions without detracting from the novelty thereof. For example, we have shown the use of a small amount of a weak acid, i.e., citric acid, for the purpose of further stabilizing the composition against contaminants and the like. Additional materials may be added to the compositions to improve the handleability of the resulting sheets, such as anti-tack agents, anti-static agents, anti-fingerprint agents, fillers, and the like.

In general, the components selected for use in making a heat marking sheet should be lightly colored, solid and non-hygroscopic, and otherwise stable at normal conditions of temperature and humidity. Further, the cocrystal adduct should be dissociable at the desired activation temperature in the range of 50150 C. The particular hydroxyphenol compound and basic organic amino nitrogen compound are accordingly to be selected with reference to these criteria on the basis of known or readily ascertainable characteristics. The dissociation temperature of a co-crystal adduct, hence the activation temperature of a copy of recording sheet, may be effectively determined by placing a coated sheet, prepared according to the earlier examples, in contact with various heated surfaces for periods of at least one second and measuring the lowest temperature at which a visibly distinct color change occurs in the heated area.

' As shown in the examples, the various dispersed coating materials will normally be applied in a vehicle containing at least a sufiicient amount of a flexible resinous material to bind the particles to the coating mass or backing sheet. This binder system may, of course, vary with the selection of co-crystal adduct and it is anticipated that the choice of a preferred binder will be but a matter of routine experimentation. It will be noted, however, that the binder system chosen must be one which does not dissolve and dissociate the co-crystal adduct. We prefer to use the aqueous binder systems containing such resins as polyvinyl alcohol, hydroxyethylcellulose, carboxymethylcellulose, gums, and the like when employing the relatively water-insensitive hydroxy fiavan adducts, while the bisphenol adduct compositions will generally be dispersed in solutions of lower aliphatic hydrocarbon solvents containing resins such as polyisobutylene, polyvinylbutyral, polyvinylacetate, and the like.

The relative amounts of the several materials should be such as to provide mutual reaction, with a total amount sufiicient to produce a distinct contrasting mark when the sheet is heated. These quantities may vary considerably, but may be readily determined by routine experimentation. Generally, however, an amount of the co-crystal adduct which will upon dissociation provide an amount of basic organic amino nitrogen compound equal to at least about five percent of the weight of the heteropoly phosphomolybdic complex compound is preferred.

The backing sheet may be any of numerous thin sheet materials in addition to glassine paper. Where, in thermographic copying processes, front printing is to be employed, i.e., reflex printing with the copy sheet interposed between the radiant heat source and the graphic original, the backing should be transparent to radiant heat, but for direct reproduction or recording purposes any flexible backing sheet would be suitable.

In the present description of our invention we have thus far primarily considered its application in sheets for use in copy and recording processes employing heat as a means of activation. It should be noted here that our novel use of the co-crystal adducts provides for additional practical means for activating the sheet, that is, initiating the color forming reduction of the heteropoly compounds.

Since the reduction reaction by which the color is formed I in our sheets may proceed at normal temperatures and since the reaction is not primarily dependent upon increased temperatures, any means by which the co-crystal adduct may be dissociated to yield the basic organic amino nitrogen compound and the hydroxyphenol compound will usually be effective to activate the sheet. Such activating means might include electromagnetic radiations, high frequency sonic vibrations, and other types of directed excitation energies. The application of various solvents to the sheet might also be employed to cause color formation. As previously noted, the use of such solvents must be avoided in the preparation of the sheet.

The accompanying drawing is a fragmentary view in cross-section illustrating schematically the heat sensitive sheet of this invention.

This invention has been described with reference to its preferred embodiments, but it is contemplated that modifications will readily occur to those skilled in the art and familiar with this disclosure, and that such modifications may be made without departure from the scope of this invention.

What we claim is:

1. A copy and recording sheet responsive to a temperature in the range of 50150 C. to produce a visibly contrasting mark by the formation of a colored compound comprising a thin flexible backing and a coating thereon comprising, in intimate physical association,-a solid phosphomolybdic complex heteropoly compound, and a solid potentially reactive reducing agent therefor comprising a solid co-crystal adduct formed from an hydroxyphenol compound selected from the class consisting of bisphenols and hydroxyphenol substituted chromans and a basic organic amino nitrogen compound, the said co-crystal adduct being stable at normal temperatures and dissociable at a temperature in the range of 50-15 0 C. to provide the said reducing agent in reactive contact with the said heteropoly compound to initiate the formation of the said colored compound.

2. A copy and recording sheet according to claim 1 wherein the hydroxyphenol compound is a bisphenol.

3. A copy and recording sheet according to claim 1 wherein the hydroxyphenol compound is a hydroxyphenol substituted chroman.

4. A copy and recording sheet responsive to a temperature in the range of 50-150 C. to produce a visibly contrasting mark by the formation of a colored material comprising a thin flexible backing and a coating thereon comprising a composition containing a phosphomolybdic complex heteropoly compound and reactive in the presence of a basic organic amino nitrogen compound to produce colored reduction products of the said heteropoly compound and, in intimate physical association with the said composition, a normally solid co-crystal adduct formed from an hydroxyphenol compound selected from the class consisting of bisphenols and hydroxyphenol substituted chromans and the said basic organic amino nitrogen compound, the said adduct being stable at normal temperatures and dissociable in the range of 50-150 C. to release the said basic organic amino nitrogen compound into reactive combination with the said composition.

' 5. A copy and recording sheet according to claim 4 wherein the hydroxyphenol compound is a bisphenol.

6. A copy and recording sheet according to claim 4 wherein the hydroxyphenol compound is an hydroxyphenol substituted chroman.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A COPY AND RECORDING SHEET RESPONSIVE TO A TEMPERATURE IN THE RANGE OF 50-150*C. TO PRODUCE A VISIBLY CONTRASTING MARK BY THE FORMATION OF A COLORED COMPOUND COMPRISING A THIN FLEXIBLE BACKING AND A COATING THEREON COMPRISING IN INTIMATE PHYSICAL ASSOCIATION, A SOLID PHOSPHOMOLYBDIC COMPLEX HETEROPOLY COMPOUND, AND A SOLID POTENTIALLY REACTIVE REDUCING AGENT THEREFOR COMPRISES A SOLID CO-CRYSTAL ADDUCT FORMED FROM AN HYDROXYPHENOL COMPOUND SELECTED FROM THE CLASS CONSISTING OF BISPHENOLS 